1. Field of the Invention
The present invention relates to an element which emits a enhanced near-field light, as well as to an optical head applying the element.
2. Description of the Related Art
In a related-art optical disk drive and a related-art optical lithography system, respectively, recording density and the width of a pattern are limited by the size of a light spot to be used and the limit is the order of the wavelength of the laser light to be used because it is impossible to reduce the spot size to less than the wavelength in the case of far-field concentration used in these devises.
Near-field light which emitted through a minute aperture formed in a metal film has recently drew much attention as means for producing a minute light spot exceeding the limit. In the case of the near-field light, since the spot size of the light is limited by the size of the aperture only, reducing the aperture size can reduce the spot size far less than the diffraction limit.
However, in the case of a simple aperture, the intensity of emitted near-field light is known to decrease in proportion to the fourth power of a ratio of the aperture to the wavelength (see, e.g., H. A. Bethe, Theory of Diffraction by Small Holes, Physical Review, Second Series, Vol. 66, pp. 163 to 182 (1944)).
As the means of breaking through the limit and enhancing the intensity of the near-field light, the excitation of surface plasmon by irradiating a laser beam on a metal film is promising.
Namely, the field intensity of light around the aperture is enhanced by interacting irradiated laser beam to the surface plasmon resonantly, whereby the intensity of the near-field light emitted through the aperture is also enhanced. A structure for periodically forming corrugations concentrically in a metal film around the aperture has been proposed as the means of interacting the irradiated laser beam to the surface plasmon efficiently (see JP-A-2004-70288).
FIGS. 11A and 11B show a near-field light-emitting element described in JP-A-2004-70288. As shown in FIG. 11A, the near-field light-emitting element is a rectangular metal film 10 formed on a transparent medium 6. The metal film 10 has a flat first surface 10a contacting the transparent medium 6, a second surface 10b opposing the first surface 10a, an aperture 10d formed so as to penetrate through the first to the second surfaces 10a, 10b, and plural ring-shaped recessed sections 10e formed periodically in the second surface 10b around the aperture 10d. 
Here, the recessed sections 10e will be described in detail. The periodicity P of the recessed sections 10e is determined such that the product of the periodicity “P” and the refractitive index “n” of the transparent medium 6 becomes slightly smaller than the maximum wavelength λ of the laser beam propagating through the metal film 10. The width of the recessed section 10e is made smaller than the periodicity P. The width of the actual corrugated pattern is set to 0.1 to 0.6 μm, and the periodicity of the same is set to 0.4 to 2 μm. In an optimal case, the intensity of near-field light 4e emitted from the aperture 10d is reported to have been enhanced by a factor of several hundreds times of that achieved in a case of no periodic pattern. It is also reported that the greater the periodicity, the higher the rate of enhancement and that the rate of enhancement is increased as the cross-sectional profiles of corrugations are closer to rectangular in shape (see Collection of Proceedings 3, Spring Joint Lecture related to the Japan Society of Applied Physics 2004, 29p-D-10, p-1139).
However, according to the related-art near-field light-emitting element, a corrugated pattern must be formed in advance in a transparent medium in order to form a periodic pattern, which in turn makes processes complicated. Particularly, when an corrugated pattern is formed in a light-condensing plane of a solid immersion lens or solid immersion mirror, which is effective for forming near-field light, difficulty is encountered in placing the lens or mirror in a photolithography system, because the lens or mirror has a curved surface. Thus, simple processes have been sought.
Moreover, since plasmon reflectivity from d each corrugation is low plural corrugations are required in order to achieve sufficient reflection. Accordingly, a laser beam must be irradiated onto the corrugated pattern across plural corrugated periods. Therefore, the diameter of the light spot can be converged only to a size in the order of about 1 μm. In this case, the majority of light is reflected and absorbed by the metal film 10, and hence the utilization efficiency of light, i.e., the intensity of emitted near-field light in relation to the intensity of the irradiated laser beam, is low. Even when the diameter of the aperture 10d assumes a value of 0.1 μm, only a utilization efficiency on the order of 2% or thereabouts is achieved.
In particular, in the case of an optical disk drive, the utilization efficiency of light is very important. For example, in the case of a phase-change medium used in a DVD or the like, required recording power density is 1 MW/cm2 or thereabouts. In the case of an aperture having a diameter of 0.1 μm, the irradiated laser beam requires power of about 0.1 mW. Accordingly, in the case of a light utilization efficiency of 2%, the irradiated laser beam requires power of about 50 mW.
In the field of an optical disk, recording density of 1 Tb/(inch)2 is considered to be required in the future. In that case, the diameter of the near-field light must be narrowed to a size of about 30 nm, which in turn reduces the utilization efficiency further. For this reason, much higher power is required for the irradiation. If not, most of the power is not used for recording but is absorbed by the metal film or dissipated in the optical head. As a result, the metal film or the optical head is heated, which raises various problems such as thermal distortion or exfoliation of the film. When the present method is used for photolithography, similar problems arise.